Consummating ion desolvation in hard carbon anodes for reversible sodium storage

Hard carbons are emerging as the most viable anodes to support the commercialization of sodium-ion (Na-ion) batteries due to their competitive performance. However, the hard carbon anode suffers from low initial Coulombic efficiency (ICE), and the ambiguous Na-ion (Na+) storage mechanism and interfacial chemistry fail to give a reasonable interpretation. Here, we have identified the time-dependent ion pre-desolvation on the nanopore of hard carbons, which significantly affects the Na+ storage efficiency by altering the solvation structure of electrolytes. Consummating the pre-desolvation by extending the aging time, generates a highly aggregated electrolyte configuration inside the nanopore, resulting in negligible reductive decomposition of electrolytes. When applying the above insights, the hard carbon anodes achieve a high average ICE of 98.21% in the absence of any Na supplementation techniques. Therefore, the negative-to-positive capacity ratio can be reduced to 1.02 for full cells, which enables an improved energy density. The insight into hard carbons and related interphases may be extended to other battery systems and support the continued development of battery technology.

In order to obtain more accurate and fine structural information from HRTEM images, the intelligent fringe recognition method was applied to identify structural characteristics of hard carbons according to previous studies [10][11][12] .The resulting binary image after the filtering shows clearer fringe compared to the original HRTEM image.Finally, the skeletonized fringe image can be obtained after branch pruning, which was used for data statistics and analysis.For the case cycled in ether electrolytes, the interlayer spacing is mainly concentrated around 0.4 nm (Supplementary Figure 12e), which is much smaller than the case using ester electrolytes.For the fringe length, it is mainly distributed around 0.77 nm, and there is a strong peak near 1.5 nm, which is longer than the hard carbon harvested from ester electrolytes.The increased interlayer spacing and reduced fringe length can be attributed to the irreversible intercalation of Na + in hard carbon, which expands the graphene sheets and interrupts the original continuous layered structure.The force displacement curves indicate that the SEI formed in ester electrolytes has a good flexibility as the loading and unloading curves are not fully irreversible, which is typical characteristic of organics.But for the SEI formed in ether electrolytes, it shows small displacement in elastic deformation, which is closer to the mechanical properties of pristine hard carbons.and its distribution and content can be inferred from the F 1s spectra.The F 1s peak position is basically unchanged, only the intensity can see a slight decrease with etching, suggesting that the distribution of F-related organic species in the S-SEI and I-SEI is relatively uniform.The end of the table shows the equivalent circuit used for fitting EIS data, which consists of four elements, including the internal resistance (R0), the resistance of the surface films (SEI) of the hard carbon anode (RSEI), and corresponding constant phase element (CPE1), the charge transfer resistance (Rct), and corresponding constant phase element (CPE2).

Supplementary Figure 10 |
XPS spectra of hard carbons immersed in electrolytes.The C 1s (a), Na 1s (b) and F 1s (c) spectra with different etching time (0-21 min) collected from the hard carbon immersing in 1 M NaPF6-EC/DEC electrolytes.The C 1s (d), Na 1s (e) and F 1s (f) spectra with different etching time (0-21 min) collected from the hard carbon immersing in 1 M NaPF6-G2 electrolytes (without Na metal).

Figure 13 |
AFM analysis of SEI formed in those two electrolytes.AFM height images of cycled hard carbon in 1M NaPF6-EC/DEC electrolytes (a) and 1M NaPF6-G2 electrolytes (b).Two-dimensional AFM maps of elastic modulus AFM analysis of cycled hard carbons in 1 M NaPF6-EC/DEC electrolytes (c) and 1M NaPF6-G2 electrolytes (d).e, f, Corresponding representative force-displacement curves of selected sites in Fig. c and Fig. d.

table 3 .
Na||hard carbon half cells recorded at different aging time and corresponding equivalent circuits.The EIS fitting results of Na||hard carbon half cells at different discharge state in ester electrolytes and corresponding equivalent circuits.The end of the table shows the equivalent circuit used for fitting EIS data, which consists of four elements, including the internal resistance (R0), the resistance of the surface films (SEI) of the hard carbon anode (RSEI), and corresponding constant phase element (CPE1), the charge transfer resistance (Rct), and corresponding constant phase element (CPE2).
The end of the table shows the equivalent circuit used for fitting EIS data, which consists of four elements, including the internal resistance (R0), charge transfer resistance (Rct), constant phase element (CPE1), and the Warburg's element (Wo).

table 4 .
The EIS fitting results of Na||hard carbon half cells at different discharge state in ether electrolytes and corresponding equivalent circuits.